Satellite radio operators provide digital quality radio broadcast services covering the entire continental United States. These services offer approximately 100 channels, of which 50 or more channels in a typical configuration will provide music with the remaining stations offering news, sports, talk and data channels.
Satellite radio has the ability to improve terrestrial radio's potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHZ) of the electro-magnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are owned by Sirius Satellite Radio and 12.5 MHz of which are owned by the assignee of the present application “XM Satellite Radio Inc.”
The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters broadcast the same program content in order to improve coverage reliability. Some mobile receivers simultaneously receive signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation of multipath interference and addresses reception issues associated with blockage of the satellite signals. In accordance with XM Satellite Radio's unique scheme, the 12.5 MHZ band is split into 6 slots. Four slots are used for satellite transmission. The remaining two slots are used for terrestrial reinforcement.
In accordance with the XM frequency plan, each of two geostationary Hughes 702 satellites transmit identical program content. The signals transmitted with QPSK modulation from each satellite (hereinafter satellite 1 and satellite 2) are time interleaved to lower the short-term time correlation and to maximize the robustness of the signal. For reliable reception, the LOS signals transmitted from satellite 1 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by terrestrial repeaters. The assigned 12.5 MHZ bandwidth (hereinafter the “XM” band) is partitioned into two equal ensembles or program groups A and B. Each ensemble is transmitted by each satellite on a separate radio frequency (RF) carrier. With terrestrial repeaters transmitting an A and a B signal, six total slots are provided, each slot being centered at a different RF carrier frequency.
Although there are satellite systems such as Loran and the Global Positioning System that provide location finding capabilities, there is no current location finding capability in such a system as described above where only two satellites are used in combination with terrestrial repeaters. Thus, a need exists for a method and apparatus of determining location in a communication system having at least two satellite transmission sources and optional terrestrial transmission sources.